Membrane processes are well known in the art of separation science, and can be applied to a range of separations of species of varying molecular weights in liquid and gas phases (see for example “Membrane Technology and Applications” 2nd Edition, R. W. Baker, John Wiley and Sons Ltd, ISBN 0-470-85445-6).
Nanofiltration is a membrane process utilising membranes whose pores are generally in the range 0.5-5 nm, and which have molecular weight cut-offs in the region of 200-2,000 Daltons. Molecular weight cut-off of a membrane is generally defined as the molecular weight of a molecule that would exhibit a rejection of 90% when subjected to nanofiltration by the membrane. Nanofiltration has been widely applied to filtration of aqueous fluids, but due to a lack of suitable solvent stable membranes has not been widely applied to the separation of solutes in organic solvents. This is despite the fact that organic solvent nanofiltration (OSN) has many potential applications in manufacturing industry including solvent exchange, catalyst recovery and recycling, purifications, and concentrations. U.S. Pat. Nos. 5,174,899 5,215,667; 5,288,818; 5,298,669 and 5,395,979 disclose the separation of organometallic compounds and/or metal carbonyls from their solutions in organic media. UK Patent No. GB2373743 describes the application of OSN to solvent exchange; UK Patent No. GB2369311 describes the application of OSN to recycle of phase transfer agents, and; EP1590361 describes the application of OSN to the separation of synthons during oligonucleotide synthesis. However, there are no reports to date describing the application of OSN in strongly basic or acidic organic solvent environments.
Polyimides have been used widely to form membranes used in separation processes, particularly gas separations, and also for separations of liquids. U.S. Pat. Nos. 5,264,166 and 6,180,008 describe processes for the production of integrally skinned asymmetric polyimide membranes. These membranes are prepared as flat sheet membranes on a supporting substrate using a phase inversion technique, which results in an ultra-thin top layer of the asymmetric membrane characterised by pore sizes below 5 nm in diameter. After formation, the membranes are treated with a non-volatile conditioning agent dissolved in solvent. The conditioning agent maintains membrane properties for nanofiltration of low molecular weight solutes from organic solvents, and allows the membrane to be processed, stored and handled in a dry state. The application of these membranes to solvent recovery from lube oil filtrates are described in U.S. Pat. Nos. 5,360,530; 5,494,566; and 5,651,877. GB 2,437,519 reports membranes formed by phase inversion of polyimide solutions, followed by crosslinking of the resulting polyimide membrane, and then treatment with a non-volatile conditioning agent dissolved in solvent. However integrally skinned polyimide membranes formed by phase inversion are not stable in all solvents, even when crosslinked according to GB 2,437,519. In particular, they are not stable in strongly basic or acidic organic environments.
Polybenzimidazole membranes have been widely reported for use in gas separations and processing of aqueous fluids. U.S. Pat. Nos. 3,699,038, 3,720,607, 3,841,492, 4,448,687 and 4,693,824 report the formation of integrally skinned polybenzimidizole membranes formed by phase inversion from a dope solution. U.S. Pat. No. 3,737,402 reports the formation of polybenzimidzole membranes by phase inversion from a dope solution, followed by annealing at temperatures of at least 135° C. to improve the reverse osmosis performance of the membranes. U.S. Pat. No. 4,693,825 reports the production of polybenzimidazole membranes from a dope solution containing benzyl alcohol as an additive.
It has been reported that crosslinking of polybenzimidizole (PBI) membranes improves their chemical resistance. U.S. Pat. Nos. 4,666,996, 6,986,844, 4,734,466, and 4,020,142 all disclose methods for the crosslinking PBI. However, these methods are known to lead to a dramatic increase in the brittleness of the membranes, making them difficult to manufacture and use.